1. Field of the Invention
The present invention relates to an exhaust gas sensor diagnostic device that diagnoses response of an exhaust gas sensor, which is provided to an exhaust gas flow passage of an internal combustion engine and which senses a gas state in the exhaust gas flow passage.
2. Description of Related Art
Conventionally, exhaust gas sensors such as an A/F (air-fuel ratio) sensor, a NOx (nitrogen oxide) sensor, a PM (particulate matter) sensor and an exhaust gas temperature sensor have been known as exhaust gas sensors provided to an exhaust gas flow passage of an internal combustion engine for sensing a gas state in the exhaust gas flow passage. An engine ECU (electronic control unit) controls fuel injection quantity and EGR (exhaust gas recirculation) gas quantity based on outputs of the exhaust gas sensors and controls an engine operation state into a suitable state.
There is a case where response of the output of the exhaust gas sensor lowers as compared to the normal exhaust gas sensor when at least a part of vent holes of a sensor cover (which prevents sensor element from getting wet from water) of the exhaust gas sensor is blocked by the particulate matters or when a sensor element of the exhaust gas sensor degrades, for example.
Delay in the response of the exhaust gas sensor is not problematic when the engine operation state is constant and the output of the exhaust gas sensor does not change. However, when the engine operation state shifts from a steady state to a transient state or from the transient state to the steady state, the engine operation state sensed from the output of the exhaust gas sensor having the lowered response delays from a state sensed with the normal exhaust gas sensor.
In this case, if an actual output of the exhaust gas sensor is corrected based on a deviation between an estimated output of the exhaust gas sensor estimated from the engine operation state and the actual output of the exhaust gas sensor without taking the lowering of the response of the exhaust gas sensor into account, there is a possibility that erroneous correction is performed.
There is a possibility that deterioration of emission and increase of a combustion noise are incurred if the fuel injection quantity, the EGR gas quantity and the like are controlled based on a deviation between the state of the exhaust gas, which is obtained from the output of the exhaust gas sensor having the lowered response or from the erroneously-corrected output of the exhaust gas sensor, and a target state of the exhaust gas.
Therefore, for example, a technology described in Patent document 1 (JP-A-2007-309103) estimates an output value of an oxygen concentration sensor (as exhaust gas sensor) at the time when response of the oxygen concentration sensor has lowered. The technology determines the lowering of the response of the oxygen concentration sensor by comparing the lowered estimate (i.e., estimate corresponding to lowered response) with an actual output value.
The technology of Patent document 1 can determine a magnitude relationship between the lowered estimate and the actual output value of the oxygen concentration sensor by comparing the lowered estimate and the actual output value of the oxygen concentration sensor. That is, the technology can determine whether actual response of the oxygen concentration sensor is higher or lower than the lowered estimate by comparing the lowered estimate and the actual output value of the oxygen concentration sensor. However, the technology cannot diagnose whether the response of the oxygen concentration sensor has lowered significantly or slightly. That is, the technology cannot diagnose a lowering degree of the response.